Closed Loop Inlet Pressure Control for Melt Pump Extrusion Tech Notes

In order to obtain an extruded profile of consistent quality, it is necessary to keep both the extruder output and melt viscosity constant. In most cases, the extruder output varies to sosme degree with time, as does the melt quality. The magnitude of output instability or surging is difficult to measure on a continuous basis, although changes in a related parameter, melt pressure can be measure. No direct relation between output and melt pressure exists due to melt viscosity fluctuations, but in practice die pressure fluctuations do correlate with output changes.

(1). A head pressure variation of 1% is roughly equivalent to an extruder output variation of 1-3% depending on the rheological behavior of the polymer melt

(2). The pressure instability observed may be the result of solid bed breakup, feed irregularities, or a number of temperature instabilities. In the extruder, melting and pressurization occur simultaneously, and inconsistency in either process will influence the other

(3). There is not doubt properly designed extruder screws are capable of delivering melts to the extrusion die at relatively constant pressure and temperature, reportedly with 20-150 psi and 1-2oF respectively

(4). However, in many cases, surging is a significant problem, particularly at high screw speeds. No simple solutions exist as the extruder screw's function is a complex one. It must fee, melt, mix and pump the polymer

(5). Unfortunately, screws with good temperature and pressure stability often have low outputs.

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